Methods for using a laser source to erode surfaces of workpieces are known.
It is an object of the present invention to provide an improvement in such methods and apparatus whereby such techniques can be applied to sensitive surfaces and in particular to objects in which it would be undesirable to affect underlying layers.
In the field of medicine a known technique for the treatment of certain forms of myopia is surgically to remove a segment of the collagen sub-surface layer of the eye, to reshape the removed segment as by surgical grinding, and to restore the reshaped segment in the eye. The eye heals by reformation of the outer cellular layer over the reshaped collagen layer. Alternatively a layer of the cornea is opened up as a flap, an artificial or donor lenticular implant is inserted under the flap, and the flap is sutured up again.
It is a further object of this invention to provide an improved and less traumatic method and apparatus for reshaping the cornea of the eye.
It is also known to perform corneal transplants in which a section of the cornea from a donor eye is transplanted to the eye of a patient. The transplanted donor corneal insert has to be stitched to the patient's eye, and quite commonly accidental overtightening of some of the stiches introduces refractive errors into the cornea following the operation. At present, there are two methods of removing these refractive errors. For both it is necessary to wait until the patient's eye has healed. In one method the transplant operation is then performed again. In the other method, relaxing incisions are made in the cornea to change its shape.
It is a further object of this invention to provide an improved and less traumatic method and apparatus for remedying refractive errors introduced during corneal transplant operations.
It is known to treat corneal ulcers by scraping the ulcerous material off the cornea. However, this tends to spread the ulcerous cells. Additionally, the scraping tends not to leave a smooth underlying surface, and the corneal surface layer which reforms subsequently may not be entirely clear and may also tend to become ulcerated again.
It is a further object of this invention to provide a method and apparatus for removing corneal ulcers which does not spread ulcerous cells across the eye and which leaves a smooth surface after removal of the cells.
The use of a laser beam as a surgical tool for cutting incisions, a so-called "laser scalpel", has been known for some time (see for example U.S. Pat. No. 3,769,963 to Goldman L et al). In 1980, a study was made of the damage which might be inflicted on the corneal epithelium by exposure to recently developed Excimer lasers (see Taboada et al "Response of the Corneal Epithelium to KrF excimer laser pulses" Health Physics 1981 Volume 40 pp677-683). At that period, surgical operations on the cornea were commonly carried out using diamond or steel knives or razors, and further such techniques were still being studied (see for example Binder et al "Refractive Keratoplasty" Arch. Ophthalmol. May 1982 Vol 100 p 802). The use of a physical cutting tool in corneal operations, and the insertion of an implant under a flap, continue to be widely practiced and techniques further developed up to the present day (see for example "Refractive Keratoplasty improves with Polysulfone, Pocket Incision" Ophthalmology Times, July 1, 1986).
It has been suggested in European Patent Application No. 0151869 of L'Esperance, to perform controlled ablative photodecomposition of one or more selected regions of a cornea using a scanning action on the cornea with a beam from an Excimer laser. Because of the scanning action, it is necessary for L'Esperance to bring his laser beam to a small spot, typically a rounded-square dot of size 0.5 mm by 0.5 mm. There is no suggestion that the spot size or shape should be varied, or controlled other than to produce the above-mentioned standard size and shape, and indeed, in the L'Esperance system any such spot size variation would be highly undesirable since the effects of the laser beam on the cornea are intended to be controlled by controlling the scanning path.
L'Esperance suggests that myopic and hyperopic conditions can be reduced by altering the curvature of the outer surface of the cornea by repeatedly scanning the cornea with an Excimer laser beam having this standard, small spot size but varying the field which is scanned during successive scans, so that some areas of the cornea are scanned more often than others. In this way it is claimed that the surface can be eroded by different amounts depending on the number of times they are scanned by the spot. L'Esperance also suggests making uniform-depth removals of material from the cornea to provide a recess for the reception and location of a corneal transplant. Additionally, he suggests that certain severe myopic and hyperopic conditions may be treated with a reduced removal of tissue by providing the outer surface of the cornea with a new shape having Fresnel-type steps between areas of the desired curvature.
In practice, complex apparatus is required to cause a laser beam to scan with the precision required if the eroded surface is to be smooth. Thus if successive sweeps of a scan overlap there will be excessive erosion in the overlap area, whereas if they fail to meet a ridge will be left between the sweeps. The compression of the Excimer laser beam to a small spot will increase the beam energy density, which will tend to exacerbate these problems. It is not clear that L'Esperance has found a suitable scanning system, since in one embodiment he attempts to control the laser beam by a magnetic field.
Additionally, the scanning method is inherently time-consuming even with highly refined techniques and apparatus, since the laser beam is only eroding a very small part of the total area to be treated at any given moment.
Furthermore, such a scanning system will tend to cause rippling effects on relatively soft materials such as corneal tissue.
It is therefore a further object of the present invention to provide a method and apparatus for eroding a surface using a laser which does not require scanning of the area of the surface to be eroded.